How to use select_circle in a modal operator to select faces in edit mode?
Ok I figured it out..
from:
(Is there a way to set the radius of the new circle select tool by python script)
def get_3d_area_and_region(self):
    self.view_3d_area = None
    self.view_3d_region = None
    for area in bpy.context.screen.areas:  # Cycle through all the avaliable areas
        if area.type == &quot;VIEW_3D&quot;:  # if area is called VIEW_3D
            for region in area.regions:  # Cycle through all the regions in that area
                if region.type == &quot;WINDOW&quot;:  # if the region is WINDOW
                    self.view_3d_area = area  # save the area and region in variables
                    self.view_3d_region = region
                    return
    print (&quot;Could not find area/region&quot;)

and then
override = context.copy()
            override['area'] = self.view_3d_area  
            override['region'] = self.view_3d_region
            bpy.ops.view3d.select_circle(override, x=event.mouse_region_x, y=event.mouse_region_y, radius=100) 

I still don't really understand why the context area information of my own operator is not enough and I still have to query all the areas extra. Oh well, some day I will know more..:)